A related art, commercially available thread-chasing tool includes a basic body having an insert seat, which is delimited by a plane bottom surface and two side support surfaces, which converge at an angle of 60° toward an inner clearance space, one of which is long and forms a primary side support surface, the other one of which is shorter and forms a secondary side support surface. At each one of the three corners, the turning inserts of the tool are formed with a tooth in which the cutting edge is included. More precisely, the cutting edge includes a pair of opposite part edges in order to generate the flanks of the thread groove, and possibly also a third, central part edge in order to generate a groove bottom having a certain width. When the turning insert is fixed in the basic body by a tightening element in the form of a screw, not only the underside of the insert is kept urged against the bottom of the insert seat, but also two of the side surfaces of the insert are kept urged against the two side support surfaces of the insert seat. The urging of the turning insert against the side support surfaces is possible as a consequence of the screw being spring biased by having an inherent elasticity and being eccentrically placed in relation to the hole in the turning insert. In such a way, the screw can transfer a rectilinear motion to the turning insert, which wedges up the side surfaces of the turning insert against the converging side support surfaces of the insert seat.
When the active cutting tooth of the turning insert enters the workpiece at the entrance of the thread to be made, the workpiece is first entered by one of the part edges of the tooth, a force being applied to the turning insert, which aims to turn the insert in one of the directions of rotation around the tightening screw. As soon as the opposite part edge also has entered the workpiece so that a full thread is turned, the cutting forces on the two part edges will become essentially equally large. When the cutting tooth finally leaves the workpiece at the exit of the thread, the first part edge is detached before the other. This means that the interaction of forces is inverted so far that a force is applied to the turning insert, which aims to turn the insert in the opposite direction of rotation around the screw.
A disadvantage of the known turning tool is that the fixation of the turning insert may become unstable and unreliable, among other things as a consequence of the fact that the only active structure that, besides the tightening screw, counteracts turning of the turning insert is the two converging side support surfaces in the insert seat. Thus, the underside of the turning insert and the bottom of the insert seat are plane and smooth, and therefore they lack the capability of locking the turning insert. Considering the rule that small convergence angles give a good wedging effect, and vice versa, the convergence angle of 60° of the side surfaces of the turning insert is comparatively large. Even if the stability problems are moderate at the entering of the turning insert into the workpiece, as the cutting forces aim to turn the turning insert in such a way that the forces essentially are carried by the primary, long side support surface, the forces become marked when the turning insert leaves the workpiece and is subjected to forces that primarily are to be carried by the short, secondary side support surface. In the latter case, the risk is large that the side contact surface of the turning insert slides along the co-operating, secondary side support surface during tensioning of the tightening screw, dislodging the turning insert.
If the turning insert on and off is dislodged from the desired, predetermined position thereof, the result becomes an impaired dimensional accuracy of the thread turned. Moreover, in polar cases, breakdowns may occur.
Another disadvantage of the known turning tool is that the contact length along the primary side support surface of the insert seat and the side surface of the turning insert has to be long. This means that the freedom of the designer to form the teeth on turning inserts for turning threads significantly is limited, because the greater part of the available space is required for the side support surfaces. Hence, it follows that the teeth have to be made small.
Related art thread-chasing tools of the kind in question are disclosed in, for instance, European patent documents EP 0119175 and EP 0178273.
U.S. Pat. No. 5,810,518 discloses a turning tool for, among other things, thread turning, which makes use of a triangular turning insert, which is mounted on the basic body via an interface that includes elongate and straight, female-like and male-like engagement members. However, in this case, the turning insert is locked in relation to the basic body via serration surfaces, which individually include a plurality of ridges and grooves, which are in engagement with each other simultaneously. In addition, the turning insert lacks support for the side surfaces of the insert. For these reasons, the manufacture of the tool is complicated, at the same time as possible form defects of the turning insert may give rise to poor machining precision of the tool.
The present invention aims at obviating the above-mentioned shortcomings of related art turning tools, and at providing an improved turning tool. Therefore, an object of the invention to provide a tool, the turning insert of which is kept stably fixed in the desired, predetermined position of the insert under varying stresses on the insert.
Another object of the invention is to provide improved stability by simple and few ways, which guarantees a continued simple and cost-effective manufacture of the turning insert as well as the basic body. Among other things, the requirements of precision in the design of the insert seat in the basic body should be moderate.
Yet another object of the invention is to increase the freedom of the designer to form space-requiring teeth having a complicated shape at the corners of the turning insert.